The present invention relates to an X-ray CT (Computed Tomography) apparatus and an X-ray CT imaging method, and more specifically to an X-ray CT apparatus and an X-ray CT imaging method both having no needless X-ray irradiation and capable of reducing an exposure range of a subject at a helical scan (including a variable pitch helical scan or a variable speed helical scan).
There has heretofore been known an X-ray CT apparatus wherein when a helical scan is performed, a collimator located on the front side as viewed in a linearly-moved direction of the helical scan restricts the position of an end surface of an X-ray cone beam on the front side as viewed in the linearly-moved direction at the start of X-ray irradiation in order to prevent exposure of the front side as viewed in the linearly-moved direction rather than a linearly-moved range in which one desires to collect projection data, and the collimator located on the rear side as viewed in the linearly-moved direction restricts the position of an end surface of the X-ray cone beam on the rear side as viewed in the linearly-moved direction at the completion of the X-ray irradiation in order to prevent exposure of the rear side as viewed in the linearly-moved direction rather than the linearly-moved range in which one desires to collect the projection data (refer to, for example, a patent document 1).
[Patent Document 1] Japanese Patent Publication Laid-Open No. 2002-320609
In the conventional X-ray CT apparatus, an X-ray spread to the front side as viewed in the linearly-moved direction rather than the linearly-moved range in which one desires to collect the projection data, is shielded or blocked by the collimator, and an X-ray spread to the rear side as viewed in the linearly-moved direction rather than the linearly-moved range in which one desires to collect the projection data, is shielded by the collimator, thus reducing an exposure range of a subject at the helical scan.
However, a problem arises in that the X-ray at each shielded portion results in needless X-ray irradiation. It is also difficult to perform the collimator control with satisfactory accuracy by real-time control. There is a possibility that an error in X-ray irradiation will occur due to a control error. Hence problems associated with image quality and needless exposure have arisen.